The present invention relates to a cathode for an electrochemical cell. In particular, the cathode of the present invention is for use in a high temperature rechargeable electrochemical cell.
It is known that an effective way of handling a cathode in the form of a powdered mixture (as described in GB 2,182,194 A, for example) is to form granules, as in GB 2,281,436 B. The resulting granules are free flowing and prevent segregation of the different constituents of the electrode. Typically, the granules contain nickel, iron, sodium chloride, sodium fluoride, sodium iodide and aluminium.
Although granular electrodes are easier to handle than powdered electrodes, it has now been found that when they are assembled into an electrochemical cell, gaps or spaces are created between the granules thereby reducing the capacity density of the resulting electrode. While it has been found that it is possible to increase the capacity density by increasing the ratio of sodium chloride to metal in the granules, because the density of the sodium chloride is much less than the density of iron or nickel, the bulk density of the resulting granulate is reduced. This method also causes an undesirable increase in the utilisation of the metal because the increase in sodium chloride in the granules reduces the amount of metal in the electrode. The higher the utilisation of the metal, the less metal available as a conducting backbone and the more rapid the rate of degradation of performance of the electrochemical cell. For example, increasing the amount of sodium chloride by a factor of 1.5 in the granules reduces the bulk density of the granules by 10% and increases the capacity density by only 14%, with an associated increase in the utilisation of the metal from 0.3 Ah/g to 0.46 Ah/g.
As described in GB 2,281,436 B, aluminium is added to an electrode for generating porosity as it forms sodium chloroaluminate and thus promotes full charge on the maiden charge. In addition, the aluminium generates a small amount of sodium to wet the anode compartment side of the solid electrolyte separator, and also provides some overdischarge capacity. However, it has now been found that the presence of aluminium in the powdered mixture used to form granules results in granules that are more hygroscopic i.e., the powdered mixture becomes more humidity-sensitive after the compaction and granulation process. Without wishing to be bound by theory, this is thought to be because when the aluminium powder is present in the powdered mixture, the granulation process disrupts the oxide film on the aluminium making the aluminium much more reactive. The resulting granules take up water at a much greater rate than the blended powder-mix.
Since the presence of even small quantities of moisture is wholly undesirable, it is important to prepare the granules in dry conditions. Even small quantities of moisture cause vigorous effervescence with the liberation of gaseous hydrogen chloride upon the dispersion of a molten salt electrolyte about the granules.
Thus, there remains a need for an improved cathode which solves one or more of the problems described above.
The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgment that the document is part of the state of the art or is common general knowledge.